Nanotechnology-Enabled Sensors

372 Chapter 7: Organic Nanotechnology Enabled Sensors
7.2 Surface Interactions
The study of various surfaces (including the surfaces of nanostructures)
and understanding how to manipulate those surfaces are the basis of the
development of organic sensors. Knowledge about the interactions of target
molecules with organic sensors are of great importance as the performance
(i.e. the selectivity, durability and stability) of the sensors depends on
these interactions.
If the sensor is a surface-type sensor (or affinity sensor), its active area
is prepared by the addition of sensitive and selective layers. In the bulktype
sensors, we are dealing with surfaces of nanostructures or nanoparticles
within the bulk of the transducer or in the bulk of the target media that
are manipulated in order to selectively bind to desired target molecules. In
both cases of surface and bulk type sensors, the importance of the surface
interactions is obvious. In order to sense biocomponents, such as proteins
and DNA, it has to be immobilized on the surface of an affinity sensor or
on the surface of nanostructures within the bulk of a bulk-type sensor.
These surfaces have to become both selective and sensitive to the target
component.
Literature on the development of organic thin films and sensitive/selec-
tive surfaces is quite extensive and many issues relating to maintaining
their activities, optimization and tuning have been studied and addressed.
1-3 Different approaches are possible for the development of such
surfaces. The approach taken depends on the transducer type, the nature of
organic components, the surface chemistry of the device, the way the device
is used, and the nature of the samples.
In this section, the most common processes for the creation of surfaces
suitable for interactions with biomolecules and organic materials will be
presented. Such interactions can be physical, chemical or a combination
of both. The type of interactions can cover a wide range from very strong
covalent bonds to weak van der Waals interactions. Furthermore, two
major surface fabrication strategies: self-assembly and layer-by-layer
techniques will be presented.
7.2.1 Covalent Coupling
When atoms share pairs of electrons between them covalent bonds are
formed. Many organic derivatives possess different surface functional
groups such as amines, thiols, and carboxylic acids. These functional
groups can be used to form covalent bonds with the functional groups on
the target organic molecules such as proteins (Fig. 7.). A covalent bond